Method and apparatus for controlling fluence in computed x-ray imaging

ABSTRACT

The present invention pertains to a system and method for X-ray imaging wherein a targeted fluence at the detector for projection images can be achieved at a plurality of projection angles around the imaging subject by control of exposure times implemented during image acquisition. Exposure time for a second projection image may be determined by the fluence in a first projection image, and in a third projection image by the fluence in a second projection image, where projection images are acquired within two degrees of one another. An acquisition parameter calculation can be configured to calculate acquisition parameters, such as said exposure times, to achieve the targeted fluence in projection images and can be coupled to a rotation controller that implements the acquisition parameters by controlling a relative angle between the imaging subject and X-ray image acquisition device.

RELATED U.S. APPLICATION

This application is a continuation application claiming priority fromthe co-pending U.S. non-provisional patent application Ser. No.14/714,083, Attorney Docket Number TRT-12-C1, entitled “METHOD ANDAPPARATUS FOR CONTROLLING DOSE IN COMPUTED X-RAY IMAGING,” with filingdate May 15, 2015. U.S. non-provisional patent application Ser. No.14/714,083 claims priority to U.S. non-provisional patent applicationSer. No. 13/896,208, Attorney Docket Number TRT-12, entitled “SYSTEM ANDMETHOD FOR SAVING TIME AND DOSE IN COMPUTED TOMOGRAPHY,” with filingdate May 16, 2013. U.S. non-provisional patent application Ser. No.13/896,208 claims priority to U.S. provisional patent application, Ser.No. 61/774,289, Attorney Docket Number TRT-12, entitled “System andMethod for Improved Contrast-to-Noise in Computed Tomography,” withfiling date Mar. 7, 2013, all of which are hereby incorporated byreference in their entirety.

FIELD OF THE INVENTION

The present invention pertains to systems and methods for computedtomography. The present invention also pertains to systems and methodsfor computed tomography in industrial metrology and dental applications.

BACKGROUND

Computed tomography (CT) is an X-ray imaging modality that providing theability to non-invasively gain information about three-dimensionalstructure within an object. While computed tomography has been pervasivein medical, diagnostic, and intraoperative applications since itsdevelopment in 1972, it has also been tailored to industrialapplications mostly since the 1990's. Industrial metrology systems aredescribed, for example, in U.S. Pat. Nos. 5,027,378 and 4,422,177.

Industrial computed tomography systems, e.g., such as coordinatemeasuring machines (CMM), can allow objects and work pieces, which mayhave external and internal structure made of metal, plastics,composites, and other materials, to be investigated without disassemblyor destruction. Non-destructive measurement or investigation can beparticularly useful in a final stage of industrial manufacturing, wherea work piece may require validation of precise dimensions and internalconstruction. This capability can also be useful for later stageanalysis, such as failure prediction or analysis.

To allow precise measurements of a work piece or detection of finedefects, such as cracks, vacancies, or similar features, industrial CTsystems may utilize microfocus X-ray sources for their ability toprovide micron-level resolution. Microfocus X-ray tubes can berelatively limited in operating power relative to sources having largerfocal spots due to the enhanced localization of the thermal load in theanode. Potentially high densities or large sizes of work pieces inconjunction with relatively lower X-ray tube power can cause scan timesin industrial metrology to be significantly longer than for medical CTscans. Overly long scan times can limit throughput of a manufacturingoperation.

Computed tomography systems have also been tailored for dental imagingapplications in recent years. Dental computed tomography images canprovide information including the structure and density of teeth and jawbones and the positioning of nerves. This information can be useful forvarious types of surgical planning as well as improving the ability tocomplete a pre-surgical assessment of whether a patient is a goodcandidate for dental implants.

The utility of computed tomography images in metrology, dental, andother applications can be dependent not only on image resolution butalso on image quality aspects such as the contrast-to-noise ratio (CNR)of the image, presence of artifacts that obscure image details, or otheraspects that affect the ability to resolve true internal features of theimaging subject. These aspects of image quality can be determined by avariety of factors, including the X-ray tube power and total scan time.What is needed is a system and method of providing high image qualitywithin scan time and tube power constraints of a CT application.

SUMMARY

The present invention pertains to a system and method for X-ray imagingwherein a targeted detector fluence for projection images can beachieved at a plurality of projection angles around the imaging subjectby control of exposure times implemented during image acquisition. Theplurality of projection angles may comprise at least 200 unique relativeangles between the X-ray image acquisition device and an axis of theimaging subject spanning at least 180 degrees around the subject.Targeted exposure times can be determined by simulating expecteddetector fluence for projection images or by acquiring a preliminaryimage data set with uniform exposure times and less than 50% of theX-ray source power to be used for image acquisition or by the full X-raysource power to be used for image acquisition. Alternatively, exposuretime for a second projection image may be determined by the detectorfluence in a first projection image, and in a third projection image bythe detector fluence in a second projection image, where projectionimages are acquired within two degrees of one another. The subject orimage acquisition device can be continuously rotated during imageacquisition, such as by determination and implementation of a rotationalvelocity sequence, or images can be acquired with the subject andimaging device being stationary at each projection angle. An X-ray imagecan be reconstructed from acquired image data.

An acquisition parameter calculation can be configured to calculateacquisition parameters, such as said exposure times, to achieve thetargeted detector fluence in projection images and can be coupled to arotation controller that implements the acquisition parameters bycontrolling a relative angle between the imaging subject and X-ray imageacquisition device, such as by controlled rotation of the imagingsubject stage or the image acquisition device. A feedback loop may beprovided between the rotation controller and the element that itcontrols.

These and other objects and advantages of the various embodiments of thepresent invention will be recognized by those of ordinary skill in theart after reading the following detailed description of the embodimentsthat are illustrated in the various drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings and in whichlike reference numerals refer to similar elements.

FIG. 1 is a diagram representing one system of an embodiment of thepresent invention wherein an object can be imaged on a rotary stage,rotation of the rotary stage being controlled according to measured orexpected attenuation characteristics of the object.

FIG. 2 is a diagram representing a system of an embodiment of thepresent invention wherein image acquisition hardware can be rotatedaround an object being imaged, rotation of the rotary stage beingcontrolled according to measured or expected attenuation characteristicsof the object.

FIG. 3 is a flow diagram showing a number of the embodiments describedabove that comprise measuring or predicting the attenuationcharacteristics of an object at different projection angles, determininga target amount of exposure for each projection angle, and implementinga pattern of acquisition that achieves these target amounts of exposure

FIG. 4 is a diagram representing one of many possible velocity profilesin embodiments of the present invention.

FIG. 5 is a diagram representing an embodiment of the present inventioncomprising real-time prospective determination of acquisition timesachieving targeted levels of fluence at each projection angle.

DETAILED DESCRIPTION

Reference will now be made in detail to embodiments of the presentinvention, examples of which are illustrated in the accompanyingdrawings. While the invention will be described in conjunction withthese embodiments, it will be understood that they are not intended tolimit the invention to these embodiments. On the contrary, the inventionis intended to cover alternatives, modifications and equivalents, whichmay be included within the spirit and scope of the invention as definedby the appended claims. Furthermore, in the following detaileddescription of embodiments of the present invention, numerous specificdetails are set forth in order to provide a thorough understanding ofthe present invention. However, it will be recognized by one of ordinaryskill in the art that the present invention may be practiced withoutthese specific details. In other instances, well-known methods,procedures, components, and circuits have not been described in detailas not to unnecessarily obscure aspects of the embodiments of thepresent invention.

The image quality of a computed tomography image can be related to theprojection image in a set having highest noise level across allprojections in the set. High noise in a single or few projections in aCT data set can result in streaking artifacts or other undesirableeffects in reconstructed images, even if many other projections in thedata set have relatively low noise. The noise level of each projectionmay be related to the amount of photon attenuation occurring through theobject at that projection angle; noise can scale as ˜1/√{square rootover (N)}, N being the number of photons per unit of detector area, e.g.the detector fluence. X-ray photon flux along a given path through anobject may be predicted by the equation:

I=I ₀ e ^(−∫μ(x)x dx)

where I is the X-ray photon flux, if the attenuation coefficients (μ) atpositions (x) through the object and the X-ray photon flux of theincident x-ray beam (I₀) are predetermined. Fluence can be determined bythe integral or sum of detector flux in a given area over the exposuretime. Projections of an object in which the paths through the object areparticularly thick, comprise material with high attenuationcoefficients, or that have some combination of these properties, mayexhibit more noise relative to projections in which paths are thinner orcomprise materials with lower attenuation coefficients.

In existing implementations of computed tomography, acquisition hardwareor, in some cases, the object being imaged, is rotated at a constantangular speed during imaging. In these systems, projections can beexposed with more or less photons depending on their attenuationcharacteristics by x-ray tube current modulation. However, tube currentcan only be modulated up to a predetermined maximum current withoutexceeding performance limits of the tube. X-ray tube current modulationcan therefore be tailored to reducing radiation dose, e.g., todecreasing exposure in a few predetermined projections, but not easilyto improving the image quality of a final image. In embodiments of thepresent invention exposure of projections can be modulated based onattenuation characteristics in a manner to decrease noise in highlyattenuating regions and to improve the quality of the finalreconstructed image.

In embodiments of the present invention, detector fluence at variousprojection angles may be modulated by controlling the exposure time ofeach projection angle or the number of exposures taken at thatprojection angle. However, these embodiments do not necessarily requireincreasing the total scan time to accommodate increased exposure timesat relatively attenuating projections; instead, the increased time atrelatively attenuating projection angles can be compensated byimplementing decreased exposure times or fewer acquisitions atrelatively transparent projection angles. Decreasing the exposure timeat relatively transparent projection angles in these embodiments may notaffect the image quality, e.g., introduce or enhance artifacts, of thefinal reconstructed image as long as the exposure times are notdecreased below a point at which noise performance becomes worse than inthe relatively opaque regions. Decreasing exposure times for relativelytransparent projection angles may have additional benefits such asreduced occurrences of detector saturation.

FIG. 1 is a diagram representing one system of an embodiment of thepresent invention wherein an object can be imaged on a rotary stage,rotation of the rotary stage being controlled according to measured orexpected attenuation characteristics of the object. Methods ofdetermining rotation controls according to measured or expectedattenuation characteristics are discussed in more detail with respect toFIG. 3 and FIG. 5 below. Image acquisition hardware 14 can be utilizedto acquire projection images of an object mounted on a rotary stage 13.Rotary stage 13 can be any type of rotary stage including but notlimited to a mechanical-, hydraulic-, or pneumatic-bearing rotary stageor any other type of rotatable platform. Rotation of stage 13 duringimage acquisition can be controlled by a rotation controller 12,including but not limited to a DC motor, stepper motor, servomotor, wormdrive, encoder, actuator, or any similar controller or combinationsthereof. Feedback between rotary stage 13 and rotation controller 12 mayoptionally be included to validate positioning of rotary stage 13. Forexample, rotary stage 13 may be equipped with an active sensor toindependently measure the acquisition angle. A feedback loop between theactive sensor and rotation controller 12 may validate or correctpositioning of the rotary stage.

Inputs or instructions for rotation controller 12 can be determined byan acquisition parameter calculation unit 11. Acquisition parametercalculation unit 11 can be implemented in any type of computationalengine, including but not limited to a microprocessor, microcontroller,desktop computer, single- or multi-core processor, calculation engineembedded in another device, or any other computational engine orcombinations thereof. An image reconstructor 15 receiving data fromimage acquisition hardware 14 can also be implemented in a computationalengine, including but not limited to those listed above.

FIG. 2 is a diagram representing a system of an embodiment of thepresent invention wherein image acquisition hardware can be rotatedaround an object being imaged, rotation of the rotary stage beingcontrolled according to measured or expected attenuation characteristicsof the object. Similarly to the embodiment of FIG. 1, the embodiment ofFIG. 2 may comprise an acquisition parameter calculation unit 11,rotation controller 12, image acquisition hardware 14, and imagereconstructor 15. However, a stage 21 wherein an imaging subject ispositioned may be stationary, and rotation controller 12 may instead becoupled to image acquisition hardware 14. Stage 21 may be comprise aplatform, cage, or other type of support structure, e.g., configured tosupport a work piece other object. Stage 21 may alternatively comprise adental bite structure, e.g., configured for precise positioning and tocontrol motion of a human head, chair, bed, or other structureconfigured for a human imaging subject. Feedback may optionally beprovided for validation of the positioning of image acquisition hardware14.

In the embodiments of FIG. 1 and FIG. 2, image acquisition hardware 14can comprise a radiation source or sources, including but not limited tox-ray sources comprising reflection or transmission targets, microfocusx-ray sources, rotating anode sources, or any other type of x-ray beamsource. In one embodiment of the present invention, the maximum tubecurrent of the source may be between 0.1 mA and 100 mA. The maximum tubecurrent may also be between 0.5 mA and 10 mA, 1 mA and 7 mA, 1 mA and 3mA, 3 mA and 1 A, or 1 A and 5 A, inclusive, or any integer ornon-integer number of milliamps or amps within the enumerated ranges.

Image acquisition hardware 14 can also comprise a radiation detector ordetector array including but not limited to photon-counting,energy-integrating, energy-resolving, flat panel, or any other type ofradiation detector. In one embodiment, the radiation detector can be aflat panel radiation detector having side lengths between 10 cm and 100cm, inclusive, e.g., such as 20 cm or 41 cm. The flat panel detector maybe any type of direct flat panel detector, including but not limited toLi-doped Si or Ge direct flat panel detectors or amorphous Se directflat panel detectors. The flat panel detector may alternatively be anytype of indirect flat panel detector, including but not limited toamorphous Si detectors coupled with scintillating materials.

Image acquisition hardware 14 can further comprise source- ordetector-related hardware and electronics, including without limitationmechanical support structures, detector read electronics, signalprocessors or amplifiers, external power supplies, anti-scatter grids,x-ray beam filters such as bowtie filters, or any other related hardwareor electronics. Support structures may be configured to implement asource-to-detector between 20 cm and 160 cm, inclusive. Thesource-to-detector distance can, for example, be 20 cm, 40 cm, 60 cm, 80cm, or any other integer or non-integer number of centimeters within theenumerated range. The source-to-detector distance may be fixed orvariable. Rotary stage 13 or stage 21 may be positioned relative toimage acquisition hardware 14 to implement an object-to-detector, e.g.distance from the imaging subject to the detector, between 4 cm and 130cm, inclusive. An object-to-detector distance may be up to 80% of thesource-to-detector distance. The position of the stage may be fixed orvariable.

In embodiments of the present invention, scans can comprise rotation ofthe acquisition hardware or rotary stage through any set of projectionangles at least sufficient for a complete CT data set, e.g., 180° plusthe angle of a cone or fan beam. This set of angles may be between 180°and 360° around the object. This set of angles may further includebetween 180° and 200°, 200° and 230°, 230° and 270°, or 270° and 320°around the object, inclusive. The number of projections acquired duringa scan may vary across application and desired image quality. Forexample, in industrial metrology applications, work pieces can sometimesbe imaged using over 1,000 unique projection angles. However,embodiments of the present invention may also acquire between 100 and500 projections, 500 and 900 projection, or 900 and 1500 projections,inclusive, or any other number of projections within or between theenumerated ranges.

Scans may also comprise linear motion between image acquisition hardwareand the imaging subject. For example, rotary stage 13 or stage 21 of theembodiments of FIG. 1 and FIG. 2, respectively, can be moved along acentral axis between the x-ray source and detector, e.g., to complete ahelical scan. Alternatively, the x-ray source and detector can be movedalong an axis of the imaging subject. However, embodiments of thepresent invention may also be implemented without such linear motion,and a complete imaging subject, or a region of interest within theimaging subject, can be captured by a non-helical scan.

FIG. 3 is a flow diagram showing methods that comprise measuring orpredicting the attenuation characteristics of an object at differentprojection angles, determining a target amount of exposure for eachprojection angle, and implementing a pattern of acquisition thatachieves these target amounts of exposure of embodiments of the presentinvention. Attenuation characteristics of the object to be imaged fromdifferent projection angles can be measured or predicted (S31) in avariety of ways. In one embodiment, the dimensions and materialcomposition that should be exhibited by the work piece may be predictedor predetermined. For example, the goal of coordinate-measurementmetrology (CMM) or similar industrial metrology applications can bevalidation of measurements of work pieces against a template or masterwork piece. The attenuation properties of a work piece may be predictedor approximated analytically, e.g., by calculating expected photontransmission such as by using the integral equations for flux above.Attenuation properties may also or alternatively be predicted throughsimulation, e.g., by ray tracing simulations of x-ray photons through amodel of the work piece. Such properties may also or alternatively bedetermined from images acquired of a similar work piece, such as amaster work piece that is known to have the desired dimensions andcomposition of a batch of work pieces.

In some embodiments of the present invention, patterns in theattenuation properties of an object as the object is rotated may bepredicted, if not absolute attenuation properties at a specifiedprojection angle. For example, in one embodiment of the presentinvention comprising dental CT methods, a human head can be positionedat a predetermined location and angular orientation such thatapproximate levels of attenuation may be predicted from general anatomy.Additional methods, such as photographing a human subject, e.g., usingvisible light, or otherwise measuring dimensions of the head of thesubject may be utilized to further predict or approximate attenuationcharacteristics.

In another embodiment of the present invention, attenuationcharacteristics from different projection angles may be determined by apreliminary scan of the imaging subject, e.g., object or work piece. Inone such embodiment, a first scan can be acquired at a relatively highpower, which may or may not be the maximum power of the x-ray tube.Power during the first scan may, for example, be at least 5 W, 10 W, 20W, 50 W, or 100 W, inclusive, or any other integer or non-integer numberof watts between or above the enumerated values. Power during the firstscan may also, particularly if the source comprises a rotating anode, beat least 1 kW, 5 kW, 10 kW, 50 kW, or 100 kW, inclusive, or any otherinteger or non-integer number of kilowatts between or above theenumerated values. A subsequent scan or set of scans may revisit orre-expose only those projections that are determined to have noiseperformance below a predetermined threshold.

In another embodiment of the present invention, a preliminary or scoutscan can be taken at 1%, 2%, 3%, 4%, 5%, 10% of the maximum tube power,or any integer or non-integer percentage below or between the enumeratedvalues. In a further embodiment, the beam may alternatively or inaddition be collimated to a small region of the object. The beam may,for example, be collimated to 5%, 10%, 15%, 30%, or 25% of the fullbeam, e.g., of the solid angle of the beam that will be implementedduring subsequent scans, inclusive, or any other integer or non-integerpercentages between the enumerated values. The scout scan mayalternatively be acquired with the tube at full power but moving at arelatively high speed, including but not limited to at least 1 rev/s,0.75 rev/s, 0.5 rev/s, 0.25 rev/s, 10 rev/hr, 5 rev/hr, or 2 rev/hr,inclusive, or any other speed between or above the enumerated values.The scout scan may also be acquired at 1.5×, 2×, 3×, 4×, 5×, 6×, or anyother integer or non-integer factor faster than the average speed to beutilized during subsequent acquisitions. These embodiments of thepresent invention may be particularly useful in applications whereinlimiting radiation dose is desirable, such as when a human patient orliving subject is being imaged. Parameters of the scout scan may betailored such that few to no projections are exposed to enough photonsto exceed the desired photon intensity or noise performance of theimage.

Determination of target amounts of exposure for each projection angle(S32) may utilize attenuation characteristics information, e.g., asdetermined in step S31. In one embodiment of the present invention,target exposure times may be determined based on a target noise levelfor projections in a data set, e.g., a uniform noise level target. Inthis embodiment, target exposure times may be related to the square rootof the transmitted fluence at each projection angle or a similarfunction of the transmitted fluence at each projection angle. Forexample, target exposure times may be calculated such that arelationship between the target exposure times t_(i) and t_(j) of anytwo projection angles within a dataset or subset of the dataset is√{square root over (F_(i))}/√{square root over (F_(j))}=t_(j)/t_(i)where F_(i) and F_(j) are transmitted fluence values for the twoprojection angles.

In one embodiment of the present invention, fluence values that arepredicted or evaluated to determine target exposure times, e.g., F_(i)and F_(j), can be the mean transmitted fluence over the full detectorarea. In other embodiments of the present invention, such fluence valuescan be the minimum fluence value over all beam paths reaching thedetector, the minimum fluence value in a predetermined region of thedetector, the mean fluence value over a predetermined region of thedetector, the mean fluence value over a region of interest in theprojection image, or any similar measure or quantification of fluence.In one embodiment, a predetermined region in which fluence is evaluatedcan be a central region of an image, e.g., a circular, square,rectangular, ellipsoidal, trapezoidal, or any other shaped regioncomprising up to 20%, 30%, 40%, 50%, 60%, 70%, or 80% of the projectionarea. A predetermined region in which fluence is evaluated may also be auser-selected or predetermined region of interest (ROI) in theprojection images. In one embodiment of the present invention, such anROI can be selected on a preliminary or previously acquired image beforea scan. In another embodiment of the present invention, an ROI can beautomatically set using image processing or recognition methods,including but not limited to difference imaging, convolution methods,Fourier transformations, artifact identification, or any other methods.A central region or ROI may be tailored to exclude edges of a projectionimage that are over-saturated, e.g., as can occur if an object isnarrower than the field of view in some projection angles; regions ofmetal or very highly attenuating features; or other regions of an imagethat could highly skew mean fluence. Fluence through a predetermined orcentral region can be predicted or evaluated in any of the methods thathave been described, e.g., with respect to step S31, including but notlimited to by use of simulations, an image of a master work piece, ascout scan, or similar methods.

One embodiment of the present invention can further comprise acquiringprojections in a step-and-shoot mode (S37). In this embodiment, theacquisition hardware or rotary stage may be rotated in angularincrements, or steps, between each projection angle once the desiredexposure at each projection angle has been achieved. The source anddetector may be stationary during each exposure. This embodiment mayyield projection images without blurring or resultant image artifactsthat can occur when there is relative motion between acquisitionhardware and an imaging subject during acquisition.

Other embodiments of the present invention can comprise continuous,e.g., non step-wise, rotation of the imaging hardware or imaging subjectduring image acquisition. These embodiments may comprise an additionalstep S33 of calculating a velocity profile, e.g., sequence of velocitiesor velocity function, that can achieve the target amounts of exposure ateach projection angle. The calculated velocity profile can beimplemented during acquisition (S34), such as by appropriateacceleration, deceleration, or control of the rotation of imaginghardware or a rotary stage. In these embodiments, the rotation speed maybe increased across projection angles exposing relatively x-raytransparent projections and decreased across projection angles exposingrelatively x-ray opaque projections. In one embodiment, the exposuretimes of each projection can be modulated (S35), e.g., such thatexposure times may be longer during slowed periods of slowed rotationalspeeds and shorter during periods of increased rotational speeds. In analternative embodiment of the present invention, projections may beacquired at a constant rate (S36).

Modulating exposure times, e.g., as in S36, rather than number ofexposures, e.g., rather than S35, may incur relatively less noise perprojection angle; while electronic dark noise may scale with exposuretime, and hence be equal across these embodiments, there can also be anamount of noise associated with the process of reading signals from thedetector, which can be incurred each time the detector is read. Thisdetector read noise can be incurred only once per projection image ifexposure time is modulated whereas it may be incurred repeatedly ifmultiple exposures are taken for a given projection image.

In one embodiment of the present invention, variant S34 of FIG. 3 canfurther comprise grouping adjacent projections with relatively similarattenuation characteristics into subsets. The number of subsets intowhich a scan can be divided can be relatively small, e.g. less than 10,or may be larger, e.g. up to 500. The number of subsets may further bebetween 2 and 5, 5 and 10, 10 and 20, 20 and 50, 50 and 100, or 100 and250, inclusive, or any other number of subsets within or between theenumerated ranges. Target amounts of exposure can be calculated for eachsubset, e.g., rather than for each projection, such that each projectionwithin a subset is allocated an equal amount of exposure time. Thisembodiment may reduce the number of rotational speed variations duringimaging, or may smooth the velocity profile.

FIG. 4 is a diagram representing one of many possible velocity profilesin embodiments of the present invention. The velocity profile of FIG. 4can be sinusoidal, U-shape, quadratic, or otherwise smoothly varyingbetween a maximum velocity v_(max) and a minimum velocity v_(min).Velocity profiles of embodiments of the present invention can share thesinusoidal or smooth characteristics of the profile of FIG. 4 or mayhave any other shape, e.g., according to the attenuation properties ofan imaging subject.

In one embodiment, the velocity profile of FIG. 4 may be tailored fordental CT. The maximum velocity v_(max) may be implemented throughprojection angles at which photons travel through shortest or leastattenuating paths through a head, e.g., through a side profile view of ahuman head, and the minimum velocity v_(min) may be implemented throughthe projection angles at which photons travel through the longest ormost attenuating paths through a head, e.g. through a head front toback. The horizontal axis in FIG. 4 may represent degrees, radians, orany other unit of angular measure. The plot in FIG. 4 may represent a180° section of a scan.

The average velocity v_(avg) of a velocity profile in embodiments of thepresent invention may be any velocity or rotational speed between 1revolution per second (rev/s) and 0.1 revolutions per hour (rev/hr). Theaverage velocity of embodiments of the present invention for metrologyapplications may further be between 0.1 and 0.4 rev/hr, 0.4 and 0.7rev/hr, 0.7 and 1.0 rev/hr, 1.0 and 1.3 rev/hr, 1.3 and 1.6 rev/hr, 1.6and 2.0 rev/hr, 2.0 and 2.5 rev/hr, 2.5 and 3.0 rev/hr, 3.0 and 3.5rev/hr, 3.5 and 4.0 rev/hr, 4.0 and 4.5 rev/hr, and 4.5 and 5.0 rev/hr,inclusive, and any other integer or non-integer number of revolutionsper hour within or between the enumerated ranges. The average velocityof embodiments of the present invention may further be between 0.1 and0.2 rev/s, 0.2 and 0.3 rev/s, 0.3 and 0.4 rev/s, 0.4 and 0.5 rev/s, 0.5and 0.6 rev/s, and 0.6 and 0.7 rev/s, inclusive, and any other integeror non-integer number of revolutions per second within or between theenumerated ranges.

In the embodiment of FIG. 4 and similar embodiments of the presentinvention, a maximum velocity v_(max) of a calculated velocity profilemay be 5×, 4×, 3×, 2×, 1.9×, 1.8×, 1.7×, 1.6×, or 1.5× greater than anon-zero minimum velocity v_(min) inclusive, or any other integer ornon-integer factor between or below the enumerated values. For example,v_(max) may be 1.2×, 1.75×, 2.3×, 2.9×, 3.24×v_(min), or greater thanv_(min) by any other factor within the enumerated ranges. While tubecurrent modulation techniques can be limited by a maximum tube currentavailable, or by the range over which the tube can be modulated,embodiments of the present invention can be configured to enable a widerange of rotational velocities, e.g., enabling delivery of aparticularly wide range in the exposure to different projection anglesduring a single scan.

FIG. 5 is a diagram representing an embodiment of the present inventioncomprising real-time prospective determination of acquisition timestailored to achieve targeted levels of fluence at each projection angle.In this embodiment, a first step S51 can comprise determining a targetamount of fluence or target amount of noise. A first projection imagecan be acquired with a predetermined acquisition time. In oneembodiment, the first projection image may be acquired at an angleexpected, e.g., from simulation or prior knowledge, to be one of theleast attenuating projections through the imaging subject. Thepredetermined acquisition time may be any length of time between 0.01 sand 600 s, inclusive. For example, the first projection image may beacquired with an acquisition, e.g., exposure, time of 0.05 s, 0.1 s, 1s, or 60 s. The fluence achieved in S52 can be analyzed (S53) afteracquisition of the first projection image. An acquisition time for thenext projection can be increased (S55), maintained at present level(S54), or decreased (S56) depending on whether fluence in the mostrecently acquired projection fell below, at, or above the target levelof fluence. The next projection image may be acquired with the updatedacquisition time, e.g. as shown in step S57. Steps S53 through S57 maybe repeated through a full scan of an imaging subject.

The angular difference between contiguous projection images orprojection angles in the embodiment of FIG. 5 and other embodiments thathave been described may be any angular increment between 0.05° and 2°,inclusive, and any other integer or non-integer increment within theenumerated range. The rotation between projections in this embodimentcan, for example, comprise rotation by 0.25°, 0.3°, 0.33°, 0.4°, 0.42°,0.5°, or 0.6°.

Modification of a subsequent acquisition time, e.g., as in step S55 orS56, can be implemented in a step-and-shoot mode of acquisition byvarying the time between steps, e.g., the dwell time at a projectionangle. Modification of subsequent acquisition time may also beimplemented in other embodiments of the present invention, such as byincreasing the velocity of a rotating object or acquisition hardware,e.g., to decrease acquisition time, or decreasing the velocity of arotating object or acquisition hardware, e.g., to increase acquisitiontime. An implemented relationship between updated acquisition time andvelocity modification can be linear or have any other function. Arelationship between target acquisition time and velocity can bedetermined given the maximum and minimum achievable velocities of thesystem, acceleration capabilities of the system, total scan time, tubecurrent, and similar parameters.

In step S53, fluence may be analyzed in one or more of a variety ofmanners. For example, minimum fluence, mean fluence, minimum fluence ina predetermined region, or mean fluence in a predetermined region may beevaluated against a target value. A predetermined region in whichfluence is evaluated, e.g., in step S53, may be a central region of animage, e.g., a circular, square, rectangular, ellipsoidal, trapezoidal,or any other shaped region comprising up to 20%, 30%, 40%, 50%, 60%,70%, or 80% of the projection area. A predetermined region in whichfluence is evaluated may also be a user-selectable or predeterminedregion of interest (ROI) in the projection images. In one embodiment ofthe present invention, an ROI may be selected by the user on a single orsmall number of projection images acquired before a scan. In anotherembodiment of the present invention, an ROI may be automatically setbased on a single or small number of projection images based on imageprocessing or recognition methods, including but not limited todifference imaging, convolution methods, Fourier transformations,artifact identification, or any other methods. A central region or ROImay also or alternatively be tailored to exclude any over-saturatedregions, e.g., edges, of a projection image, or regions of metal or veryhighly attenuating features that could highly skew mean fluence.

In some computed tomography applications, image acquisition hardware,e.g., x-ray sources, detectors, or other electronics, can be rotatedaround the object to be imaged at very high speeds. This type of CTacquisition is of particular utility in the medical field, e.g., fordiagnostic or pre- or post-surgical CAT scans. In medical CT, minimizingthe total time of the scan may be of high importance, e.g., for timesensitivity of medical procedures or for reducing image artifacts due topatient motion. Many technologies have therefore been developed torotate acquisition hardware at very high speeds, such that a full scancan be completed in some cases in as little as approximately 300 ms.However, in many other CT applications, time constraints may belessened. Embodiments of the present invention may be particularlyuseful for metrology, dental, or other applications wherein total scantime does not need to be kept below 1 s.

Total acquisition time in embodiments of the present invention can varywidely across applications and attenuation properties of imagingsubjects. Total acquisition time may be as little as 0.5 s, 1 s, or 5 s,inclusive, and any integer or non-integer number of seconds between orabove the enumerated values. Embodiments of the present invention havingthese relatively low total acquisition times may be particularly usefulfor dental CT, small-scale metrology, or similar applications. Totalacquisition times may also be as long as 0.5 h, 1 h, 2 h, or 3 h,inclusive, or any integer or non-integer number of hours between orbelow the enumerated values. Systems with relatively long totalacquisition times may be particularly useful for large-scale orhigh-resolution metrology or similar applications. Computedtomography-based coordinate measurement methods can provide accuratevalidation or evaluation of irregularities for industrial metrologyapplications. Relatively long total scan times can be acceptable inindustrial metrology, as high image quality can be important for precisecoordinate measurement and validation, and work pieces may be highlyattenuating.

Dental CT is also a growing field for computed tomography and may havelessened time constraints relative to medical CT since a dental patientcan be positioned in a manner less prone to voluntary or involuntarymovement. The patient may, for example, be provided with a givenstructure for the duration of a scan to ensure accurate and stillpositioning. Involuntary motions, e.g., cardiac motion or other organmotion, are also relatively absent from dental imaging. Otherapplications may exist wherein a living subject can be imaged with CTunder lessened time constraints.

Embodiments of the present invention can be implemented within conebeam, fan beam, point source, or any other CT frameworks. Embodiments ofthe present invention may be particularly useful for cone beam systemsas these systems may utilize flat panel detectors. Cone beam CT systemsmay also operate at longer scan times or slower acquisition rates thanother CT frameworks.

Embodiments of the present invention may also be implemented withinother applications, including but not limited to tomosynthesisapplications, such as digital tomosynthesis or tomosynthesis formammography. In these applications, projection images can be acquiredthrough a set of angles comprising less than 180 degrees around thesubject. Images can, for example, be acquired through an angular setcomprising between 5 degrees and 60 degrees, 10 degrees and 55 degrees,15 degrees and 50 degrees, or 20 degrees and 45 degrees, inclusive, orany other integer or non-integer number of degrees within or between theenumerated ranges. A number of projections acquired over this angularset can be relatively less than in computed tomography procedures; forexample, a set of projection images can comprise between 3 and 30projections, 5 and 25 projections, 7 and 20 projections, or 9 and 15projections images can be acquired, inclusive, or any other number ofimages. Exposure times for projections in this set can be controlledaccording to any of the embodiments of the present invention that havebeen described.

The foregoing descriptions of specific embodiments of the presentinvention have been presented for purposes of illustration anddescription. They are not intended to be exhaustive or to limit theinvention to the precise forms disclosed, and many modifications andvariations are possible in light of the above teaching. The embodimentswere chosen and described in order to best explain the principles of theinvention and its practical application, to thereby enable othersskilled in the art to best utilize the invention and various embodimentswith various modifications as are suited to the particular usecontemplated. It is intended that the scope of the invention be definedby the claims appended hereto and their equivalents.

What is claimed is:
 1. A method of acquiring an X-ray image of an objectcomprising: positioning said object in field of view of an X-ray imageacquisition device; selecting a set of a plurality of unique projectionangles between said X-ray image acquisition device and an axis throughsaid object, wherein said set spans at least 45 degrees around saidobject; selecting a targeted X-ray fluence at a detector region for eachof said projection angles; determining a targeted total exposure timefor each of said projection angles corresponding to said targeted X-rayfluence; implementing said plurality of targeted total exposure times insaid X-ray image acquisition device over said set of projection anglesto acquire an image data set; and reconstructing an X-ray image fromsaid image data set.
 2. The method of claim 1 further comprising:increasing said targeted exposure time with at least one of saidprojection angles based on increase of attenuation at said projectionangle.
 3. The method of claim 1 further comprising: reducing saturationof said detector by reducing said targeted exposure time with at leastone of said projection angles.
 4. The method of claim 1 furthercomprising: adjusting said targeted exposure time based on attenuationcharacteristics information.
 5. The method of claim 1 furthercomprising: determining attenuation characteristics at each of saidprojection angles.
 6. The method of claim 1 further comprising:controlling total exposure times by modulating individual exposure timeof each exposure.
 7. The method of claim 1 further comprising:predicting attenuation characteristics at each of said projectionangles.
 8. The method of claim 1 wherein said region is a user-selectedregion of interest.
 9. The method of claim 1 wherein said region is apredetermined region of interest.
 10. The method of claim 1 furthercomprising: excluding edges of a projection image that areover-saturated.
 11. The method of claim 1 further comprising: adjustingsaid targeted total exposure times at said projection angles dependingon fluence in a previously acquired projection.
 12. The method of claim1 wherein said region is a full detector area.